The present invention relates to a lead frame and, more particularly, to a lead frame in which a stage portion for mounting a semiconductor chip thereon is arranged at a level lower than that of the arrangement surface of lead portions.
In a lead frame used in the step of assembling a semiconductor chip, the pads of the semiconductor chip mounted on a stage portion and lead portions for electrically connecting the semiconductor chip to an external circuit are connected to each other by a wire bonding scheme in which wires are individually bonded. In place of the wiring bonding scheme, in recent years, bonding using a TAB (Tape Automated Bonding) tape has been performed.
FIG. 4 shows a conventional lead frame and particularly shows a part corresponding to one semiconductor chip, and FIG. 5 shows a part of a conventional lead frame bonded using a TAB tape when viewed from a side direction of the semiconductor chip. Referring to FIG. 4, reference numeral 20 denotes each of long guides for holding an entire lead frame; 21, a stage portion for mounting a semiconductor chip thereon; and 22, a pair of bar-like support portions, integrated with the stage portion 21, for fixing the position of the stage portion 21. Each of the support portions 22 is constituted by a support bar for only fixing the stage portion 21, a ground lead used for, in addition to fixing the stage portion 21, connection to the ground pad of a semiconductor chip, or the like. Reference numeral 25 denotes each of a plurality of leads connected to the pads of the semiconductor chip, and reference numeral 29 denotes each of tie bars for connecting the lead portions 25 to each other to fix them. The above portions are integrally formed by punching out a thin metal plate.
Referring to FIG. 5, reference numeral 6 denotes a semiconductor chip; 27, a plurality of leads of a TAB tape; 27a, inner lead portions of the leads 27 connected to the pads of the semiconductor chip 6; 27b, outer lead portions of the leads 27 connected to lead portions 25 of the lead frame; and 28, support films, e.g., each consisting of a polyimide film, of the TAB tape. The plurality of leads 27 are adhesively held by the support films 28.
In the lead frame in FIG. 4, the stage portion 21 is fixed to the guides 20 by the support portions 22, and the larger number of lead portions 25 arranged to surround the stage portion 21 are bound by the tie bars 29 and fixed to the guides 20. Some of the lead portions 25 are obliquely formed, i.e., have crank-like shapes. At a portion where the arrangement of the lead portions 25 is shifted from the arrangement of the pads of the semiconductor chip 6, the crank-like lead portions are necessary. When a degree of integration of the semiconductor chip 6 is increased to increase the number of pins extracted from the semiconductor substrate 6, necessity for the crank-like lead portions is further increased. A plurality of crank-like lead frames are generally connected to each other, and the crank-like lead frames are separated from each other after mounting the semiconductor chips 6, connection using the TAB tapes, processing such as resin sealing, or the like.
As shown in FIG. 5, when bonding is to be performed to the lead frame using a TAB tape, pads 6a formed on the upper surface of the semiconductor chip 6 mounted on the stage portion 21 of the lead frame are connected to the inner lead portions 27a, respectively, and the lead portions 25 of the lead frame are simultaneously connected to the outer lead portions 27b, respectively. That is, simultaneous bonding can be performed. The stage portion 21 is fixed at the same level as that of the arrangement surface of the lead portions 25 by the support portions 22. That is, the lead frame is formed such that all the portions of the lead frame are formed at the same level. For this reason, in order to connect the leads 27 to the lead portions 25 of the lead frame, the leads 27 of the TAB tape must be bent such that the level of the outer lead portions 27b is lower than that of the inner lead portions 27a by the thickness of the semiconductor chip 6. When the leads 27 are bent by an amount of the thickness of the semiconductor chip 6, this bending operation may cause positional errors in the outer lead portions 27b. In this case, the positional errors of the outer lead portions 27b make accurate positioning of the outer lead portions 27b very difficult during bonding, thereby disabling the simultaneous bonding.
In order to solve the above problem, the following scheme is considered. That is, the stage portion of the lead frame is arranged at a level lower than that of the arrangement surface of the lead portions to cause the leads of the TAB tape to be slightly bend downward. FIG. 6 shows a part of a conventional lead frame in which a stage portion is arranged at a level lower than that of the arrangement surface of lead portions when viewed from a side direction of the semiconductor chip, and FIG. 7 shows the portion of the lead frame when viewed from the upper direction of the semiconductor ship.
Referring to FIG. 6, reference numeral 31 denotes a stage portion of the lead frame for mounting a semiconductor chip 6; 32, a bar-like support portion for supporting the stage portion 31; 35, a lead portion; 37, a lead of a TAB tape; 37a, an outer lead portion of the lead 37 of the TAB tape; and 38, a support film of the TAB tape. Reference symbol t1 denotes the thickness of the stage portion 31, and reference symbol t3 denotes the thickness of the bent portion of the support portion 32. Note that the thickness of the portion of the support portion 32 except for the bent portion and the thickness of the lead portions 35 are t1.
Referring to FIG. 7, reference numeral 32a denotes the support portion after a positional error caused by bending the support portion 32 downward, and reference numeral 39 denotes a tie bar.
In this lead frame, as shown in FIG. 6, the support portion 32 having one end connected to the stage portion 31 is bent downward to arrange the stage portion 31 at a level lower than that of the arrangement surface of the lead portion 35, thereby preventing the outer lead portion 37b of the lead 37 of the TAB tape from being bent. However, in this prior art, when the support portion 32 is bent downward, a drawing phenomenon in which the thickness t3 of the bent portion of the support portion 32 is smaller than the thickness t1 of other portions of the lead frame such as the stage portion 31 occurs. Since the degree of this phenomenon varies depending on each support portion 32 connected to the stage portion 31, the stage portion 31 connected to the support portion 32 may be inclined, or a positional error may occur in the stage portion 31. As a result, the positional relationship between the pads of the semiconductor chip 6 and the lead portions 35 of the lead frame is vertically or horizontally shifted.
As shown in FIG. 7, when a part of each of the crank-shaped support portions 32 having one end connected to the stage portion 31 is bent downward, a drawing phenomenon in which the support portion 32 is shifted in the lateral directions (horizontal directions) occurs. That is, since the other end of each of the support portions 32 is connected and fixed to the tie bar 39, a force acts on the support portion 32 such that the support portion 32 is shifted to the position of the support portion 32a indicated by an alternate long and short dashed line in FIG. 7 using the connection portion between the support portion 32 and the tie bar 39 as a support. When the force shifts the support portion 32 to the position denoted by the reference numeral 32a, especially when the support portion 32 is used as a ground lead, a positional error occurs between the ground pad of the semiconductor chip 6 and the bonding portion of the ground lead (support portion 32) to be connected to the ground pad through the TAB tape.
The conventional lead frame is arranged as described above. For this reason, when the crank-like support portion is bent downward to arrange the stage portion of the lead frame at a level lower than that of the arrangement surface of the lead portions, the support portion is shifted in the directions (horizontal directions) perpendicular to the extending direction of the support portion. Therefore, when the support portion is used as a ground lead, a positional error occurs between the ground pad of the semiconductor chip 6 and the bonding portion of the ground lead (support portion) to be connected to the ground pad through the TAB tape. As a result, a problem, i.e., simultaneous bonding by the TAB tape becomes difficult, is posed. In addition, the positional error of the support portion may cause the support portion to be brought into contact with the lead portion adjacent to the support portion.